Dosage Calculations Using Dimensional Analysis

Anyone can do it!

Outcomes:Understand the learning components needed for student success in performing dosage calculations.Utilize dimensional analysis to calculate dosage.Potential resources for teaching dimensional analysis in the classroom.

Ways to calculate dosage:Formula Method

VH

DAmount to Give

Challenge: Must remember the extra step of changing D and H to same unit of measurement. Also may have extra variables such as body weight, times per day, etc.

Ways to calculate dosage:Ratio and Proportion

Linear Ratio and Proportion

Fractional Ratio and Proportion

500mg : 1 tab :: 1 g : x tabsProduct of the means equals the product of the extremes.

xtabs

g

tab

mg 1

1

500

The products of cross multiplication in a proportion are always equal

Dimensional AnalysisDimensional Analysis involves setting up a series of fractions.

All information is in one problemStudents do not have to remember multiple stepsStudents ask four simple questions to set up their dimensional analysis problem.

Necessities for Student Success in Dosage Calculations

Review of basic math functions – fractions and decimals

AddingSubtractingMultiplicationDivision

Students must memorize equivalents

The 4 main questionsWhat is the order?What is on hand?Do you need any equivalents?Where are you going? (or what are you being asked to give?)

Start with conversions

240 mg = ______ grains

240

1

mg

Start with conversions

240 mg = ______ grains

240

1

1

60

mg grain

mg grains

1

4

4

Order: Lasix 40 mg PO dailyDose on Hand: 80 mg/10 ml

What is the order?

dose

mg

1

40

Order: Lasix 40 mg PO dailyDose on Hand: 80 mg/10 ml

What is on hand?

dose

mg

1

40mg

ml

80

10

Order: Lasix 40 mg PO dailyDose on Hand: 80 mg/10 ml

Do you need any equivalents?

dose

mg

1

40mg

ml

80

10

Order: Lasix 40 mg PO dailyDose on Hand: 80 mg/10 ml

Where are you going? (or what are you being asked to give?)

mlmg

ml

dose

mg________

80

10

1

40

Order: Lasix 40 mg PO dailyDose on Hand: 80 mg/10 ml

Begin by cross canceling labels

mlmg

ml

dose

mg________

80

10

1

40

Order: Lasix 40 mg PO dailyDose on Hand: 80 mg/10 ml

Labels left should be “what you are looking for”

mlmg

ml

dose

mg________

80

10

1

40

Order: Lasix 40 mg PO dailyDose on Hand: 80 mg/10 ml

Do the math

mlmg

ml

dose

mg________

80

10

1

40

1

2

55

1

What about problems with different units of measurement?

Order: digoxin elixir 0.05 mg PO dailyLabel: Digoxin 50 mcg per mlHow many milliliters will you give?

dose

mg

1

05.0mcg

ml

50

1

mg

mcg

1

1000 ml

1

20

1

Complex Problems

Order: Augmentin 12.5mg/kg PO every 12 hours. Child weighs 22 lbs. Pharmacy sends a 75 ml bottle of Augmentin labeled 125mg/5ml. How many milliliters will you pour from the bottle?

dosekg

mg

/

5.12

mg

ml

125

5

lbs

kg

2.2

1

.1

22

pt

lbs ml

5275

1375

1

22

2.2

1

125

5

1

5.12

5

What about IV problems?Order: 250ml 5% D/W IV in 2.5 hours.Tubing package reads 15 gtts/ml.How many gtts/min will you adjust the IV?

hrs

ml

5.2

250ml

gtts

1

15

.min60

1hr

min___

gtts

25150

3750

60

1

1

15

5.2

250

25

Physicians order Daypro 1200 mg bid.

mg

cap

dose

mg

600

1

1

1200

1

2

cap2

Order: Estratab 1250mcg daily

mcg

mg

mg

tab

dose

mcg

1000

1

25.1

1

1

1250tab

11250

1250

1000

1

25.1

1

1

1250

1

The order reads: Administer 1000cc of D5NS over 10 hrs. The drop factor on your tubing is 15 gtts/cc.

How many gtts/min?

hours

cc

10

1000

cc

gtts

1

15

min60

1hourgtts/min

1

4

1

25

25

or

25600

15000

60

1

1

15

10

1000

Practice Problems

Your patient may receive Dilaudid 3 mg IM q3h for pain. Dilaudid is supplied in 1-ml ampules containing 4 mg. How many milliliters will you administer?

3

1

mg

dose

1

4

ml

mg ml¾= 0.75

The physician orders heparin 2500U subq. You have heparin, 5000U per ml. How many ml will you administer?

2500

1

u

dose

1

5000

ml

u ml

1

2

1

2

Your patient needs morphine gr 1/6 SQ stat for chest pain. You have a 20 ml vial labeled 15mg/ml. How many milliliters will you give?

1 6

1

/ gr

dose 1

15

ml

mg

60

1

mg

gr ml

1 6 60

15

60 6

15

10

150 67

/ /.

x

0.67

A patient with tachycardia has an order for Brevibloc to be started at 50 mcg/kg/min. The concentration is Brevibloc 5 g in 500ml of D5W. The patient weighs 176 pounds. How many ml/h should the IV pump be programmed for?

50mcg

kg / min

500

5

ml

g ml/hr

176

1

lbs

pt

.

.

1

2 2

kg

lbs.

1

1000

mg

mcg

1

1000

g

mg

60

1

min

hour

50

1

500

5

176

1

1

2 2

1

1000

1

1000

60

1

.

1

2

1

251

401

80

1

2

1

12

24

24

Resources for teaching Dimensional Analysis

IV Calculations site

 http://www.geocities.com/Heartland/Valley/3049/iv/clcltns.html

 Math tutorials

 http://www.utep.edu/nurs4710/tutorials/

 IV drip rate problems and dosage calculation problems

 http://www.dalesplace.net/iv.htm

 

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